The present invention relates to a sensor device having a piezoelectric element for measuring the viscosity, the density, the concentration, etc., of a fluid.
Up to now, in order to measure the viscosity, the density, the concentration, etc., of a fluid, a sensor device using a vibrator, such as a piezoelectric element, has been used. FIGS. 20A and 20B show an example of the sensor device of this type, in which FIG. 20A is a perspective view of the sensor device, and FIG. 20B is a partially enlarged cross-sectional view of the sensor device taken along a line VI--VI. The sensor device 40 includes a base body 12 having a thin vibrating portion 14. Onto one surface of the vibrating portion 14 is fixed a piezoelectric element 20 having a piezoelectric film 22 and a pair of electrodes 24a and 22b which are disposed to be in contact with the piezoelectric film 22, and the other surface of the vibrating portion 14 faces a space (surrounded space) 16 defined in the interior of the base body.
Also, introduction holes 18 that communicate with the space 16 are defined in the vicinity of a tip of the sensor device 40 in such a manner that a fluid flowing into the space 16 from the introduction holes 18 is led to the vibrating portion 14. The piezoelectric element 20 is covered with a protective cover 17 so as not to be in direct contact with the fluid.
In the sensor device 40 thus structured, a fluid to be measured is allowed to flow into the space 16 from the introduction holes 18 so as to be in contact with the vibrating portion 14. In this situation, a voltage is applied to the piezoelectric film 22 to vibrate the vibrating portion 14, as a result of which in the case where the viscosity of the fluid is large, the amplitude of the vibrating portion 14 becomes small whereas in the case where the viscosity of the fluid is small, the amplitude of the vibrating portion 14 becomes large, thereby being capable of measuring the viscosity of the fluid by detecting a current corresponding to that amplitude. Also, in the case where the viscosity of the fluid correlates with the concentration of the fluid or the density of components in the fluid, the concentration and the density of the fluid can be measured.
As one application of the above-described sensor device, there is a case in which the sensor device is built in a lead storage battery to measure a change in the density of an electrolyte. In other words, since a sulfuric acid aqueous solution is used as the electrolyte in the lead storage battery, and the viscosity of the sulfuric acid aqueous solution correlates with the density thereof, the density can be detected by the above sensor device. Also, since the density of the electrolyte changes depending on the charge and discharge of the battery, the charge and discharge state of the battery can be notified by measuring a change in the density.
FIG. 21 shows a state in which the above sensor device 40 is built in the lead storage battery. A porous sheet (separator) 34 is interposed between a pair of electrodes 32a and 32b of the battery, and the sensor device 40 is embedded in the porous sheet 34. The sensor device 40 in this state is accommodated within a battery case, and the electrolyte is injected thereinto in such a manner that the electrolyte is infiltrated into the porous sheet 34 so as to be fully supplied between the electrodes 32a and 32b, and the electrolyte that percolates from the porous sheet 34 enters the introduction holes 18, flows into the space 16 and is then led to the vibrating portion 14.
By the way, in order that the electrolyte is allowed to flow into the space 16 as described above, it is necessary that an air existing in the space 16 is exhausted to the exterior simultaneously, that is, the air is replaced by the flow-in electrolyte. However, in the conventional sensor device 40, when the sensor device 40 is built in the battery as shown in the figure, an opening end of the introduction holes 18 that communicate with the space 16 is in a state where it is in contact with the porous sheet 34. This makes it hard to discharge the air within the space 16, and the entrance of the electrolyte to the introduction holes 18 is irregular. As a result, there arises such a problem that it is difficult to replace the air within the space 16 with the electrolyte.